The classic radiograph or "X-ray" image is obtained by situating the object to be imaged between an X-ray emitter and an X-ray detector made of photographic film. Emitted X-rays pass through the object to expose the film, and the degree of exposure at the various points on the film are largely determined by the density of the object along the path of the X-rays.
It is now common to utilize solid-state digital X-ray detectors (e.g., an array of switching elements and photosensitive elements such as photodiodes) in place of film detectors. The charges generated by the X-rays on the various points of the detector are read and processed to generate a digital image of the object in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the imaged object, and so on.
During the digital imaging process, the image is generally not produced directly from the detector reading. Instead, the detector reading is processed to produce a cleaner image. In particular, the image is usually processed to eliminate the "offset", which arises owing to the charge state of the detector prior to the time the exposure is made. The qualities of the offset are determined by the detector's current leakage, temperature, background radiation, and a variety of other factors. The offset is desirably eliminated from the detector reading to provide better image quality.
A common method of eliminating the offset is illustrated in FIG. 1, which illustrates the radiographic imaging process over time. The detector is read at periodic time intervals t.sub.r so that the image is regularly updated. The intervals t.sub.r are spaced by the intervals t.sub.e, which represent the desired exposure time as set by the X-ray technician. When a technician requests an exposure at time T.sub.a the X-ray system activates the emitter as soon as the next full interval t.sub.e occurs. The detector is then read at the next interval t.sub.r to obtain the exposure reading, which includes the preexisting offset of the detector. The detector is allowed to settle for the following interval t.sub.e and the detector is then read at the next interval t.sub.r to obtain an approximate measure of the offset. In FIG. 1, the interval t.sub.e between the exposure reading and the offset reading is also designated by t.sub.w, the "wait" time between the two readings. The image is then produced by subtracting the offset reading from the prior exposure reading, and perhaps subjecting it to other image processing algorithms as well. An alternative process is illustrated in FIG. 2, where the readings made prior to submission of an exposure request are stored for possible use as offset readings. When an exposure is requested at time T.sub.a, the reading taken during the interval t.sub.r prior to the exposure interval t.sub.e /wait interval t.sub.w is used as the offset reading, and the subsequent read interval t.sub.r is used to obtain the exposure reading. The offset reading is then subtracted from the subsequent exposure reading.
The imaging schemes of FIGS. 1 and 2 result in good image quality, but they suffer from a significant drawback: when an exposure is requested, an access time t.sub.a is incurred before the exposure is actually made, and this period t.sub.a could be as long as approximately t.sub.e +t.sub.r. This is illustrated in FIGS. 1 and 2 by the exemplary request time T.sub.a, wherein a technician requests an exposure just after an interval t.sub.e begins; the X-ray system must wait for the next full interval t.sub.e prior to activating the emitter. The access time t.sub.a prior to the start of the exposure could be substantial, particularly since t.sub.e could be as long as 2 seconds or more. This is inconvenient, especially where an exposure is desired during a particular well-defined time period. The inventors feel that in general, a high-quality X-ray system should be able to initiate an exposure within approximately 0.7 seconds after being requested to do so (i.e., t.sub.a .ltoreq.0.7 seconds), and should provide the processed final image within 5 seconds of the request.
FIG. 3 then illustrates an alternate imaging scheme which was developed to reduce the delay (i.e., the access time t.sub.a). Detector readings are taken at time intervals t.sub.r which are spaced by small time intervals t.sub.s. When an exposure is requested at time T.sub.a an exposure is made for interval t.sub.e at the end of the current t.sub.s +t.sub.r cycle (e.g., at the end of the current period t.sub.r in the case where exposure is requested during t.sub.r, or at the end of the next period t.sub.r in the case where an exposure is requested during t.sub.s). Thus, the delay prior to an exposure may be significantly reduced because the access time t.sub.a will never exceed t.sub.s +t.sub.r (where t.sub.s is small, and generally less than t.sub.e). After the exposure interval t.sub.e, the exposure reading is taken during period t.sub.r. The detector is then allowed to settle for another time interval t.sub.e /wait interval t.sub.w to return it to an approximation of its pre-exposure state, and the offset reading is taken during the next interval t.sub.r. The image may then be produced by subtracting the offset reading from the exposure reading, and perhaps subjecting it to other image processing algorithms.
While the imaging scheme of FIG. 3 eliminates the undesirably long access time, it suffers from the drawback that it results in increased image artifacts, presumably because the offset reading taken after exposure is an inexact representation of the true pre-exposure offset. Unlike the imaging scheme of FIG. 2, this cannot be corrected by taking the offset reading prior to making the exposure since the exposure time t.sub.e, which will generally be set by automatic exposure controls, is not known prior to submission of the exposure request at time T.sub.a.
Thus, in prior digital radiographic systems, one has been forced to choose between (1) an unacceptably long potential access time t.sub.a prior to the time X-ray exposures are made; or (2) greater image artifacts, i.e., degradation of image quality, owing to less-than-ideal offset readings.